Z.-C. Dong

Charging effects in gold nanoclusters grown on octanedithiol layers

Strong interaction of gold with the terminal sulfur atoms of dithiol molecules on Au(111) effectively suppresses the penetration of deposited Au atoms through the dithiol layer and results in the formation of homogeneous Au nanoclusters. These nanoclusters, 10–15 Au2009(σ<2u200aA) in height, spread over the surface with a density of ∼1.2×1013/cm2 for coverage between 0.25–2.5 monolayers. Decoupling of the clusters from Au(111) by the octanedithiol layer (∼12u200aA in thickness) and the small self-capacitance of these nanoparticles (10−19–10−18u200aF) make it possible to observe both the Coulomb blockade in scanning tunneling spectroscopy and the Au 4f core level shifts in x-ray photoelectron spectroscopy at room temperature. Both phenomena can be attributed to a common physical origin—e2/2C—the Coulomb energy of charged particles.

Semiconducting B–C–N nanotubes with few layers

Perfectly ordered nanotubes (NTs) displaying a limited number of defect-free B-C-N shells (typically 2-4) were synthesized from CVD C NTs and a mixture of boron oxide and gold oxide placed in a flowing N2 atmosphere at ∼1950 K. The NTs were analyzed using field emission conventional and energy-filtered (Omega filter) high-resolution electron microscopes, and electron energy loss and energy dispersive X-ray spectrometers. NTs with inner diameters of 0.9-4.0 nm were frequently assembled in bundles consisting of several tubes and extending up to 1-2 μm in length. Two-terminal transport measurements on individual B-C-N nanotube bundles were carried out in-situ in a Fresnel projection microscope. The bundles displayed semiconducting behavior with an estimated band gap of ∼1 eV.

Field emission from individual B–C–N nanotube rope

The field-emission characteristics of individual ropes made of B–C–N nanotubes were measured in situ in a low-energy electron point source microscope. The tungsten field emission tip of the microscope was used as a movable electrode, approaching the rope, and acting as an anode during field-emission measurements. The atomic structure and chemical composition of the ropes were analyzed by high-resolution transmission electron microscopy and electron energy-loss spectroscopy. The tubes assembled within the ropes typically revealed open-tip ends, a small number of layers and zigzag chirality. We found that the field-emission properties of the B–C–N nanotube ropes are competitive with conventional C nanotubes, with the expected additional benefit that the B–C–N ropes exhibit higher environmental stability.

Observation of Au deposited self-assembled monolayers of octanethiol by scanning tunneling microscopy

Au deposited self-assembled monolayers (SAMs) of octanethiol molecules have been studied by scanning tunneling microscopy. We have observed ordered structures of the molecules on both the original terraces and subsequently grown Au islands after the Au deposition. These results indicate that Au atoms penetrate through and form islands underneath the SAMs. At the initial stage of Au deposition, islands with a monatomic height grow and become larger as more Au atoms are evaporated onto the surface. The number of islands remains constant as the Au coverage increases up to approximately 0.5 ML. Above this coverage, the islands on each terrace abruptly coalesce into one network structure. The second layer starts to form after coalescence, before the first layer fully covers the surface. This unique island growth is not seen in the normal homoepitaxial growth of Au on Au(111), and is presumably attributed to both the high nucleation density of deposited atoms caused by SAMs and the relatively high diffusion of adatoms along island step edges.

Scanning tunneling microscopy and X-ray photoelectron spectroscopy of silver deposited octanethiol self-assembled monolayers

Ag deposited self-assembled monolayers (SAMs) of octanethiol (CH 3 (CH 2 ) 7 SH) have been studied using scanning tunneling microscopy and X-ray photoelectron spectroscopy (XPS). At the initial stage of Ag deposition, monatomic height islands, ∼7 x 10 1 cm 2 in density, grow at the SAMs/Au(1 I I) interface and become larger as more Ag atoms are deposited up to a full monolayer coverage of Ag. The differences of the nucleation density and the growth property between Ag and Au islands can be attributed to the higher mobility of Ag atoms and the difference of the molecular packing on these islands. XPS analysis of this structure (SAMs/Ag monolayer/Au) shows that the Ag 3d 5.2 binding energy is shifted -0.3 eV with respect to bulk Ag, the C Is binding energy is ∼0.3 eV higher than that before Ag deposition, and the S 2p 3 2 binding energy exhibits little shift before and after deposition. The origin of the shift can be explained by the change of the dipole at the interface and the electrical isolation of alkyl chains from the surroundings.

Cables of BN-insulated B–C–N nanotubes

BN-covered and insulated multiwalled semiconducting B-C-N nanotubes (NT), assembled in long ropes were produced and electrically tested. The atomic structure and chemical composition of ropes were analyzed by high-resolution transmission and energy-filtered electron microscopy. Individual ropes displayed perfect insulating performance of BN-rich outer layers and excellent field emission.

Tunneling-electron-induced molecular luminescence from a nanoscale layer of organic molecules on metal substrates

Molecular luminescence from an ultrathin layer of free-base porphyrin molecules has been generated by a scanning tunneling microscope on top of a monolayer spacer of perinone derivatives on Cu(100). Tunneling-electron-induced fluorescence spectra are in good agreement with the conventional photoluminescence data of the molecule. The dominant molecular luminescence peak becomes clear and sharp for bias voltages above ∼2.1 V. The perinone monolayer does not emit light because of quenching effects; it acts as a buffer layer to enhance the decoupling of the electronic state of the porphyrin molecules from the Cu substrate. The molecular luminescence from porphyrin is attributed to the hot electron injection excitation. These results demonstrate the feasibility of electrically driven molecular luminescence on metal substrates by a nanoscale probe.

Tunneling electron induced photon emission from monolayered H2TBP porphyrin molecules on Cu(1 0 0)

Positioning of a clean scanning tunneling microscope tip above a monolayer of free-base porphyrin (H 2 TBPP) molecules on Cu(100) is found to induce merely plasmon-mediated emission with molecular fluorescence completely quenched. The molecule acts as a spacer to increase the tip-metal substrate distance to make spectra blue-shifted. Additional broad emissions at low energies may be associated with the molecules either adsorbed onto the tip apex or on the second monolayer and might suggest the involvement of modified molecular fluorescence. In both cases, the energy transfer from molecular excited states to the metal substrate is overwhelmingly dominant, leading to enhanced plasmon-mediated emission.

Tunneling electron induced luminescence from monolayered Cu-TBP porphyrin molecules adsorbed on Cu(1 0 0)

Abstract STM-induced luminescence spectra from a monolayer of Cu-TBP porphyrin molecules indicate purely plasmon-mediated emission from the Cu(1xa00xa00) substrate. The molecule appears to enhance the emission intensity, but no direct luminescence from the molecules was observed. Such observation suggests the role of molecules acting mainly as a spacer to modify the junction geometry, specifically, increasing the tip–substrate distance to cause blue-shift of emission.

Molecular fluorescence from ZnTBP porphyrin molecular layers on Cu(100) induced by tunnelling currents

Molecular fluorescence from the surface of ZnTBP porphyrin (ZnTBPP) molecular layers on Cu(100) is induced with nanoprobe excitation in the tunnelling regime. The observed well-defined molecular fluorescence is a perfect match with the standard photoluminescence data of ZnTBPP molecules. The decoupling of the electronic state of the top layer ZnTBPP is controlled by the thickness of the molecular layers. The excitation mechanism of molecular luminescence may be attributed to the hot electron injection into the molecules in proximity to the tip apex of a scanning tunnelling microscope. This approach features simplicity, bipolar operation and good reproducibility. The research provides a new way for the integration of molecular fluorescence with a nanoprobe and the development of a nanoscale molecular light source.